Digital still cameras have been widely known as an imaging device that converts an optical image of a subject into an image signal through an image sensor such as a CCD image sensor or a CMOS image sensor, and records the image signal as a digital still image at a high resolution on a storage medium such as a non-volatile memory. Recent digital still cameras generally use an image sensor of a high pixel number, i.e. one or two million pixels, which permits recording a super fine image. On the other hand, as the number of pixels of the image sensor increases, dynamic range of the image sensor is more limited, so is the brightness range in which the gradation can be exactly reproduced. As a result, the gradation becomes flat in high-light portions or in shadow portions of an image captured from a scene or subject whose brightness is diverse, which damages the image quality.
In order to compensate for the image deterioration due to the limited dynamic range of the image sensor, some prior arts suggest making exposures a number of times per one subject while varying exposure amounts step by step, see Japanese Laid-open Patent Application Nos. 2003-223387 and 2002-135648.
According to these prior arts, however, the number of shoots per one scene is decided by the width of brightness distribution range of the scene, so that the shooting number per one scene can be so large that a large capacity storage medium is necessary for storing the image data.